The present invention relates to a process for expanding at least one cross-linkable or cross-linked blank which is intended to constitute, in the cross-linked, expanded state, all or part of an elastomeric safety support of cellular structure having closed cells, said support being intended to be mounted on a wheel rim within a tire. The invention also relates to a process for cross-linking and expanding said or each blank, a device for implementing said expansion or cross-linking/expansion process, and finally a section of cross-linked, expanded support and such a support which are obtained by this process.
The safety supports according to the invention can be used for equipping tires, for example, of machinery or vehicles of the two-wheeler, automobile or heavy-vehicle type.
The use of elastomeric safety supports having closed cells is well-known for competition tires which are intended to travel on bumpy courses of the xe2x80x9ccross-country rallyxe2x80x9d type.
These cellular supports, which are usually of substantially toric form, are supposed to permit travel following a significant drop in pressure over a distance which depends in particular on the more or less severe conditions characterizing this travel, for example, following perforation of the tire.
Such supports are generally obtained by extruding a cross-linkable, expandable rubber composition which has been subjected to thermomechanical working in order to obtain a blank, then by cross-linking and expanding the blank, the origin of the expansion being the thermal decomposition of a blowing agent which is initially present in the rubber composition.
More precisely, in a first step of thermomechanical working, the rubber composition, which comprises in particular a diene elastomer such as butyl rubber (copolymer of isoprene and isobutylene), a reinforcing filler such as carbon black, a blowing agent to permit later obtaining of the expanded cellular structure, and a cross-linking system, is kneaded.
In a third step, which is carried out in an oven, the blank thus obtained is preheated, to a temperature usually of between 70xc2x0 C. and 100xc2x0 C.
In a fourth step, performed in a mold, at least partial cross-linking of the preheated blank is effected, at a temperature usually of between 130xc2x0 and 150xc2x0 C.
In a fifth step, which is performed in an oven, the demolded blank is subjected to expansion, at atmospheric pressure and at a temperature usually of between 130xc2x0 C. and 150xc2x0 C.
Thus a cross-linked, expanded support is obtained.
French patent specification FR-A-2 095 535 describes a process for foaming and hardening an elastomeric filler material, such as polyisoprene, dimethyl-methylvinyl polysiloxane or polybutadiene, within a mounted assembly formed of a previously-vulcanised tire which is mounted on a wheel rim. This filler material is intended to equip tires intended for vehicles of the heavy-vehicle type, and the main object desired is to minimize the surface oxidation within the body of the vulcanised tire.
This object is achieved in that document by implementing a process consisting of heating in an oven the mounted assembly by means of saturated steam at a temperature of approximately 149xc2x0 C., then either removing the mounted assembly from the oven or carrying out therein a plurality of heating cycles followed by removal of the steam within the oven, in order to permit blowing of the filler material.
It will be noted that this mounted assembly is intended to form an enclosure of the mold type for the filler material which it contains, owing to the fact that the foam being blown presses on the wall of this mounted assembly.
It will also be noted that the foam which presses against the wall of the mounted assembly is the seat of local chemical reactions at the location of this wall (rim or tire), which induces heterogeneity of structure of the foam which is finally formed.
The conventional cross-linking/expansion processes which use a mold for cross-linking have one major drawback, which is linked to the density characteristics of the cross-linked, expanded supports which are obtained. In fact, the cross-linked, expanded supports which are obtained by these processes using a mold are generally characterized by an intermediate layer having a significantly higher density (usually in a ratio of 1.5) than that of the core or center of the support. There is shown diagrammatically in FIG. 5 appended to the present description a view in cross-section through such a support having, radially towards the inside of its outer surface (at the location of which there is a surface layer or skin A of the support), this intermediate layer B and said core C.
Now, experience shows that this density gradient is at the origin of increased propagation towards the core of the support of the deformations which are imposed on the latter when travelling at zero pressure, which results in likewise increased internal heating of the support, which may cause the destruction thereof within a relatively short period of travel.
The object of the present invention is to propose a process for expanding at least one partially cross-linked blank which is intended to form, in the cross-linked, expanded state, all or part of an elastomeric safety support of cellular structure having closed cells, said or each support being intended to be mounted on a wheel rim within a tire, which makes it possible to overcome the aforementioned disadvantage in relation to the use of a mold for the cross-linking.
To this end, an expansion process according to the invention is such that it consists of curing said or each blank in a bath of fluid at excess pressure which is contained in an enclosure, the temperature of said bath having a maximum value of between 110xc2x0 C. and 210xc2x0 C., and the absolute pressure of said bath having at least a value equal to or greater than 5 bar and a final pressure-relief value substantially equal to 1 bar, for expanding said or each blank such that the increase in volume thereof is unlimited with respect to said enclosure.
According to one example of embodiment of the invention, this expansion process consists in varying, discretely or continuously, said absolute pressure of said fluid between a maximum value less than or equal to 26 bar and a value less than said maximum value.
According to the invention, this expansion process consists in using a liquid fluid, such as water, or a gaseous fluid, such as hot air, steam or nitrogen, for curing said or each blank.
According to an optional characteristic of the invention, this expansion process consists of cooling said bath to a temperature less than or equal to 100xc2x0 C. and to an absolute pressure of between 1 and 26 bar, following the curing of said or each blank.
According to another characteristic of the invention, this expansion process consists of curing a plurality of blanks of linear and/or curved forms in said bath.
It will be noted that the supports or parts of supports thus obtained may have forms and dimensions which are variable according to the desired applications.
The subject of the present invention is also a process for cross-linking and expanding at least one cross-linkable, expandable blank which is intended to form, in the cross-linked, expanded state, all or part of an elastomeric safety support of cellular structure having closed cells, said support being intended to be mounted on a wheel rim within a tire.
This cross-linking and expansion process according to the invention also makes it possible to overcome the aforementioned drawback in relation to cross-linked, expanded supports, which drawback results from the use of a mold for the cross-linking, and it is such that:
said cross-linking consists of curing said or each expandable or expanded blank in a bath of fluid at excess pressure which is contained in an enclosure, such that said or each blank interacts with said bath independently of said enclosure, the temperature of said bath having a maximum value of between 110xc2x0 C. and 210xc2x0 C., and the absolute pressure of said bath having at least a value equal to or greater than 14 bar, and in that
said expansion consists of subjecting said or each cross-linkable or cross-linked blank to an expansion process according to the invention such as defined above.
It will be noted that in the present description xe2x80x9ccross-linkable blankxe2x80x9d is understood to mean a blank which can still be cross-linked, independently of prior heat treatments which may have caused the start of cross-linking. Consequently, a blank which has previously been subjected to heat treatment involving surface cross-linking is for example considered as being cross-linkable. This heat treatment may have consisted of preheating in an oven of an extruded blank and/or in the operation of forming a blank itself, in the event that it is injection or transfer molding, for example, which is being effected.
It will also be noted that this cross-linking and expansion process according to the invention makes it possible to be set free from the toric geometry of the blank which is conventionally imposed by using a mold, and that it also makes it possible to obtain cross-linked, expanded supports or parts of supports having complex geometries which would be difficult to demold under satisfactory conditions, such as for example, grooves and/or lobes and/or cutouts in any one of the directions of the support or the part of the support.
It will also be noted that this mold-less cross-linking/expansion operation for cross-linking makes it possible to obtain supports or parts of supports which do not have a parting line, which does not weaken the corresponding support or part of the support.
According to one embodiment of the invention, this cross-linking and expansion process may consist:
in a first step, of subjecting said or each cross-linkable, expandable blank to said cross-linking in order to obtain a practically cross-linked, expandable blank, then
in a second step, of subjecting said or each blank which is practically cross-linked and expandable which is obtained following said first stage to said expansion, in order to obtain all or part of said corresponding cross-linked, expanded safety support.
According to another embodiment of the invention, this cross-linking and expansion process may consist:
in a first step, of subjecting said or each cross-linkable, expandable blank to said expansion in order to obtain a cross-linkable, practically expanded blank, then
in a second step, of subjecting said or each blank which is cross-linkable and practically expanded to said cross-linking, in order to obtain all or part of said corresponding cross-linked, expanded safety support.
Advantageously, the cross-linking and expansion process according to the invention consists of using one or more blanks each of which are based on a copolymer of isoprene and isobutylene (butyl rubber or IIR).
This elastomer has in particular reduced air permeability.
According to other examples of embodiment, there could also be used for the blank(s) the halogenated, in particular chlorinated or brominated, versions of this copolymer (BIIR, or CIIR rubbers, bromobutyl and chlorobutyl rubbers respectively), copolymers of dienes and of alpha-olefins, for example terpolymers of ethylene, propylene and a diene (EPDM), polychloroprene (CR), or also a blend of natural rubber (NR) and polybutadiene (BR) in substantially identical proportions.
According to one example of embodiment of the invention, a curing device according to the invention for implementing said cross-linking and expansion process of the type comprising an enclosure which is provided with at least one opening for the introduction and extraction of said or each blank, means for receiving said or each blank, means for filling said enclosure with liquid or gaseous fluid, and heating and pressurization means for the fluid contained within said enclosure, in which said enclosure comprises:
an introduction compartment provided at its inlet with an opening for introducing into said compartment a unit of cross-linkable, expandable blanks for the curing thereof, said introduction opening being provided with a means for shutting it off,
a curing compartment provided downstream of said introduction compartment and provided at its inlet with a first mobile partition for allowing it to communicate with said introduction compartment, said curing compartment being intended to contain said heated, pressurised fluid in order to obtain a unit of cross-linked, expandable blanks, and
an extraction compartment provided downstream of said curing compartment and provided at its inlet with a second mobile partition for allowing it to communicate with said curing compartment and at its outlet with an opening to atmospheric pressure for obtaining a unit of cross-linked, expanded blanks and their extraction from said enclosure, said extraction opening being provided with a means for shutting it off,
means for alternately transferring said fluid at excess pressure from said extraction compartment towards said introduction compartment, and from said introduction compartment towards said extraction compartment.
It will be noted that these compartments make it possible to cross-link and expand units of blanks continuously, by including automated displacement of each unit within one and the same compartment and/or from one compartment to another, owing to the aforementioned openings and mobile partitions.
In fact, these partitions and openings, when in the closed position, make it possible to form locks at the location of the introduction and extraction compartments and, when said openings are in the closed position and said partitions are in the open position, enable one or the other of said introduction and extraction compartments (which is then filled with fluid) to form alternately a pressure balance with the adjoining curing compartment which is itself continuously filled with fluid.
It will also be noted that the total volume expansion of the blanks is obtained in said extraction compartment when the absolute pressure in this compartment is again made equal to atmospheric pressure (by means of said extraction opening, which is then in the open position, thus forming a pressure balance with the ambient air).
It will furthermore be noted that this cross-linking/expansion device according to the invention may be advantageously integrated in an overall process for manufacturing supports or sections of supports which is implemented continuously, that is to say, directly downstream of the forming stations, for example by extrusion or by injection, and for preheating the shaped blanks in an oven.
A curing device according to the invention may comprise any automated means suitable for controlling and checking parameters of the expansion/cross-linking process, such as temperature, pressure and the flow rate of curing water.
According to another example of embodiment of the invention, a curing device for implementing said cross-linking and expansion process, which is of the type comprising an enclosure which is provided with means for the introduction and extraction of said or each blank, means for receiving said or each blank, means for filling said enclosure with liquid or gaseous fluid, and heating and pressurization means for the fluid contained within said enclosure,
is such that said enclosure is provided with a plurality of receiving means respectively provided to receive a plurality of blanks, said receiving means being mounted adjacent to each other on a conduit intended for filling said enclosure with fluid and extending into the interior thereof.
According to another characteristic of this example of embodiment, said enclosure comprises an outlet conduit connected to means for circulating said fluid towards said means for filling the enclosure.
According to another example of embodiment of the invention, a curing device for implementing said cross-linking and expansion process, of the type comprising an enclosure which is provided with at least one opening for the introduction and extraction of said or each blank, means for receiving said or each blank, means for filling said enclosure with liquid or gaseous fluid, and heating and pressurization means for the fluid contained within said enclosure, is such that:
said means for filling the enclosure are formed of a conduit opening into an opening in said enclosure, said enclosure being provided with a means which slides hermetically on the inner face of its wall for pressurising the fluid contained within said enclosure, this enclosure also being suitable for permitting emptying of said enclosure, and that
said means for heating said fluid are mounted around said enclosure and said conduit.
It will be noted that, in this example of embodiment, the sliding means for pressurising the interior of the enclosure may be of the piston type, and that said heating means may for example comprise a coil through which a heat-transfer fluid flows, or an electrical resistor.
As for the fluid which can be used in this example of embodiment, it is preferably formed of a liquid, the boiling point of this liquid being beyond the temperature used for the curing, which may vary from 110xc2x0 C. to 210xc2x0 C.
However, it is also possible to use a gas, provided that a gas is used, the relative weight of which relative to the air is suitable to permit the pressurization of said enclosure from said filling/emptying conduit.
A section of safety support according to the invention, or such a safety support also according to the invention, are obtained by the cross-linking/expansion process referred to above, said section being formed of a cross-linked, expanded rubber composition having a cellular structure having closed cells.
Advantageously, these cross-linked, expanded sections or supports according to the invention may each comprise, radially towards the inside of their outer surface, an intermediate layer, the thickness of which is between 5% and 30% of the smallest dimension of a cross-section through said section or said support, and the density of which is less than that of the core of said section or said support.
It will be noted that this density gradient could not be obtained by the aforementioned conventional processes, that is to say with cross-linking in a mold followed by expansion to atmospheric pressure.
This intermediate layer of low density makes it possible to minimize the internal heating of the support when travelling at zero pressure.
According to another characteristic of the invention, the cross-linked, expanded sections or supports obtained each have a maximum density in a surface layer at the location of their outer surfaces, radially to the outside of said intermediate layer.
It will be noted that this surface layer, which has a high density close to that of the corresponding non-expanded blank, imparts to the support or to the section of support a surface resistance which is suited, firstly, to direct mounting on the rim and, secondly, to the repeated contacts with the reliefs of the inner face of a tire.
According to another characteristic of the invention, the cross-linked, expanded supports or sections obtained each have an average density of between 0.04 and 0.4 and, for example, substantially equal to 0.13.
It will be noted that an average density of close to 0.04 makes it possible to have a support characterized by satisfactory damping of shocks and by minimized internal heating. Such a support is particularly intended to be fitted on tires of the cross-country rally type for temporary use.
An average density close to 0.4 makes it possible to impart high structural rigidity to the corresponding support, which is particularly intended to be fitted on tires bearing heavy loads.
As for an average density close to 0.13, it makes it possible to impart to the core of the corresponding support sufficient rigidity, for example to minimize the deformations of the support during travel which are imposed by centrifugal force, when the tire is under inflation pressure and when the base of the support is connected to the rim, for example by means of a reinforcement or another means of connecting to the rim. Such a support is particularly intended to be fitted on tires of the automobile type.
According to another characteristic of the invention, the respective diameters of said cells vary on average from 0.1 mm to 2 mm, over a cross-section of said support or section.
Advantageously, said cross-linked, expanded section or support are each based on a copolymer of isoprene and isobutylene.
According to one example of embodiment of the invention, said cross-linked, expanded section or support each comprise, as reinforcing filler, a blend of 10 to 30 phr silica and 10 to 30 phr carbon black (phr: parts by weight per hundred parts of elastomer(s)).
The silica which may be used may be any reinforcing silica known to the person skilled in the art, in particular any precipitated or pyrogenic silica having a BET surface area and a CTAB specific surface area both of which are less than 450 m2/g, even if the highly dispersible precipitated silicas are preferred.
In the present specification, the BET specific surface area is determined in known manner, in accordance with the method of Brunauer, Emmett and Teller described in xe2x80x9cThe Journal of the American Chemical Societyxe2x80x9d, vol. 60, page 309, February 1938, and corresponding to Standard AFNOR-NFT-45007 (November 1987); the CTAB specific surface area is the external surface area determined in accordance with the same Standard AFNOR-NFT-45007 of November 1987.
xe2x80x9cHighly dispersible silicaxe2x80x9d is understood to mean any silica having a very substantial ability to disagglomerate and to disperse in an elastomeric matrix, which can be observed in known manner by electron or optical microscopy on thin sections. As non-limitative examples of such preferred highly dispersible silicas, mention may be made for example of the silica Ultrasil VN3 from Degussa, and the silicas Zeosil 1165 MP and 1115 MP from Rhodia.
Of course, xe2x80x9csilicaxe2x80x9d is also understood to mean mixtures of different silicas, in particular of highly dispersible silicas such as described above.
Suitable carbon blacks are any carbon blacks, in particular the blacks of the type HAF, ISAF and SAF, which are conventionally used in tires, and particularly in tire treads. As non-limitative examples of such blacks, mention may be made of the blacks N115, N134, N234, N339, N347 and N375. The mass fraction of carbon black present in the reinforcing filler may vary within wide limits, this quantity preferably being from 40% to 60%, for a mass fraction of silica of from 60 to 40%.
According to one example of embodiment of the invention, the support or section of support, which comprises a base intended to be mounted on said wheel rim and a crown intended to bear on the tread of said tire following a drop in pressure within the latter, is such that it has at least one longitudinal groove extending over said crown substantially level with the latter, in the direction of the length of said support or section of support.
According to another example of embodiment of the invention, the support or section of support is such that it has at least one longitudinal cutout in its mass, which extends in the direction of the length of said support or section of support.
It will be noted that these two examples of profiles of supports or of sections of supports, which may be obtained using the cross-linking/expansion process according to the invention, cannot be obtained under satisfactory conditions by means of the conventional processes using a mold for cross-linking. In fact, these particular profiles make the demolding operation particularly difficult.